It is clear that a new field of study is emerging where previous limitations to the quantitation of the concentration of absorptive constituents in scattering media by continuous light (CW) approaches are overcome as information on both absorption and scattering parameters become available in homogeneous tissues, and localization possibilities ameliorate problems that arise with inhomogeneous tissues.
The incidence of hypoxia/ischemia and hemorrhage in pre-term neonates is well recognized and the need for early detection of these syndromes is apparent from the current studies using ultrasound and nuclear magnetic resonance. Other applications to neonatology, particularly brain hypoxia monitoring during cardiopulmonary bypass and other surgical procedures applied to the heart and even monitoring of brain hypoxia of the infant in the birth canal as affected by uterine contractions in prolonged deliveries signify other requirements for a reliable method of quantifying oxy-hemoglobin concentrations in vivo, in real time. In adults many similar applications emerge, ranging from brain hypoxia in surgical procedures of cardiopulmonary bypass or AICD testing (ventricular defibrillation), including monitoring of oxygenation of recently transplanted, liver, pancreas, etc., to the detection of altered blood flow or lack of blood flow in chronic brain disease such as Alzheimer's, Parkinson's, and multiple infarct dementia (MID). All of these applications dictate an apparatus which can be readily applied to the exposed tissue and collect sufficient data in times as short as the few seconds that may be required to make a significant reading; time is often of the essence in clinical diagnosis and decision making. Thus calibration procedures, etc., must either be made subsequent to the measurement or the system itself should be rapidly auto-calibrating during the study.
A completely different field of applications which also reflect the need for a reliable quantitative measure of oxy- or deoxy-hemoglobin is to the exercising human body either in a confined exercise test such as rowing ergometry, bicycle ergometry, or in strength testing devices, etc., where the motion of the muscle during contraction requires that the unit be firmly attached to the overlying skin. In this case, setup time prior to the exercise should be minimal and recordings of steady state deoxygenation of the muscle bed during exercise and the transient recovery following the exercise is required. Typical applications are to the testing of national rowers, to the triathlon (swimming, running and cycling), or equally important, the training and rehabilitation of muscle function following vascular surgery and the study of muscle atrophy due to extended bed rest, geriatric conditions, or space travel.
Thus, it is clear that a device which makes rapid and reproducible readings of hemoglobin deoxygenation and hemoglobin concentration is highly desirable. To be practical, however, such a device requires a high signal-to-noise ratio and a measurement algorithm that is highly robust, with some possibilities for localization.
The brain cortex and larger muscles of the leg (Vastus Lateralis, etc.) are relatively homogeneous whilst the inner layers of the brain and the muscles of the forearm are heterogeneous. Furthermore, diseased tissue, infarcted brain, the necrotic portion of tumors represent heterogeneities that are of especial interest in themselves and indeed if included in the measurement of neighboring tissues would give erroneous values of absorption and scattering. Thus, knowledge of photon propagation in tissues and judicious placement of input/output coupling is required for accurate spectroscopy, or acquisition of data sets appropriate for construction of an image.
In principle, time-resolved spectroscopy converts the measurement of concentration or intensities by transmitted or reflected light to the measurement of photon migration time delay or path length. This enables quantitation of concentration changes in highly scattering tissues which was not heretofore possible. The characteristics of such devices and its principles are described, for example, in the co-pending patent applications referenced above.